Scanning mirror-based light projection systems are known in the field of illumination systems. US2014/0029282 discloses an example of such a system, in which the light source is a laser type light source. A scanning mirror rotatable around two orthogonal axes is actuated and receives a light signal from a primary light source to project an image on to a phosphorous element. The light radiated by the primary light source, or more specifically its luminous intensity, for example, can be modulated to project a desired image on to the phosphorous element. The phosphorous element is then arranged to perform a wavelength conversion of the light signal received from the primary light source. Consequently the phosphorous element, acting as a secondary light source, re-emits light, which when combined with the light from the primary light source produce useful white light in various directions. In this kind of system a very high overall energy efficiency can be obtained, as the wavelength conversion performed by the phosphorous element is more energy efficient than the electrical-to-optical conversion of the laser light source. According to US2014/0029282, instead of using one scanning mirror rotatable around two orthogonal axes, it possible to use two mirrors instead, each movable around one axis, where the two axes are mutually orthogonal.
FIG. 1 shows a simplified cross-sectional view of an illumination system comprising a light source 1, such as a laser light source 1, and a scanning mirror assembly 3, which comprises a movable plate 5 arranged to be rotated about axis 7 in this example. The movable plate comprises a mirror and is connected to a frame 9 by two support arms (not shown), aligned on both sides of the plate along the same axis 7. In FIG. 1, the light emitted by the light source 1 is denoted by A, while the light reflected by the mirror is denoted by B. In order to save space, and in order to obtain a reflected image free of deformations, the light source should preferably be placed directly above the mirror so that the light beam A forms an angle of incidence at the mirror surface which is close to 90 degrees with respect to the centre point of the mirror when at rest. However, in this position the light source would obstruct the reflected light, thereby creating an occluded or non-illuminated spot behind the light source. In order to avoid this occlusion problem, a free passage to the reflected light directly above the mirror centre point can be guaranteed by arranging the light source slightly offset from a position directly above the centre point of the mirror, as shown in FIG. 1. However, the light source still obstructs some of the reflected light in the position as shown in FIG. 1. Furthermore, the arrangement of FIG. 1 is also not optimal in terms of use of space.
FIG. 2 shows a similar arrangement as in FIG. 1, but in this case the light source is still further offset with respect to the centre position. Now the light source no longer obstructs the reflected signals, but the configuration has the disadvantage that it takes up even more space than the configuration of FIG. 1, and the resulting or reflected image becomes clearly deformed due to the large angle of incidence of the light A striking the surface of the mirror. It is also to be noted that the current manufacturing processes of the above type illumination systems are not optimal. For instance, the angled configuration of FIG. 1 or 2 requires a very high precision in the alignment of various components, such as the light source and the scanning mirror assembly. This naturally increases the complexity of the manufacturing process.